1. Field of the Invention
This invention relates to an ink jet recording apparatus ejecting ink from a nozzle onto a recording medium such as recording paper, thereby executing recording.
2. Description of the Related Art
Ink jet recording apparatuses of the shear mode type using a piezoelectric ceramic material are known as those of the drop-on-demand type. For example, Japanese patent publication No. 63-247051 (1988) discloses one of such ink jet recording apparatuses. FIGS. 9A to 10 show a recording head used in the ink jet recording apparatus of the shear mode type. FIG. 9A is a sectional view taken along a plane crossing the length of an ink chamber of the recording head. The recording head 21 includes a cover plate 201 and a base plate 202 opposed to the cover plate 201. A plurality of shear mode wall actuators 203 are provided between the cover plate 201 and the base plate 202. Each of the shear mode wall actuators 203 is polarized in the directions of arrows F3 and F4 in FIG. 9A. An ink chamber 205 and an air chamber 212 are alternately formed between each shear mode wall actuator 203 and the adjacent one. Each shear mode wall actuator 203 has membrane electrodes 204 and 214 formed on opposite side faces thereof respectively.
FIG. 9B is a sectional view taken along the length of the recording head 21. A nozzle plate 207 is mounted to front ends of the shear mode wall actuators 203. The nozzle plate 207 is formed with nozzles 206 communicating with the ink chambers 205 respectively. A manifold 209 is mounted to rear ends of the shear mode wall actuators 203. The manifold 209 has a filler 208 preventing ink in an ink channel 213 from penetrating the air chamber 212. The manifold 209 distributes the ink from an ink tank or ink supply into the ink chambers 205. The electrodes 204 and 214 are covered with respective insulating layers (not shown) so as to be insulated from the ink. The electrodes 214 facing the respective air chambers 212 are connected to an earth line 211. The electrodes 204 formed in the respective ink chambers 205 are connected to a head driver IC 83 for applying actuator drive signals to the electrodes 204 and 214.
The following describes the relationship between the timing for application of the drive pulse signal to the recording head and the pressure induced in the vicinity of the nozzle 206 in the ink chamber 205 by the application of the drive pulse signal. In the above-described construction, a drive pulse signal is supplied to the recording head when one record data, for example, one dot of record data is recorded. The drive pulse signal corresponding to one record data is composed of two drive pulses (multipulse). The above-mentioned drive pulse signal has a drive frequency of 10.8 kHz, for example (ejection interval of 93 .mu.sec).
The head driver IC 83 applies a first drive pulse 110a with a waveform as shown in FIG. 11 to the electrode 204. An electric field with a direction of arrows F1 in FIG. 10 is then induced in the left-hand side shear mode wall actuator 203, and an electric field with a direction of arrows F2 is induced in the right-hand side shear mode wall actuator 203. Consequently, both shear mode wall actuators 203 are subjected to piezoelectric sliding deformation so that the volume of the ink chamber 205 is increased. Pressure is decreased in the vicinity of the nozzle 206 in the ink chamber 205 such that meniscus 230 is withdrawn into the ink chamber 205 (at time T1), as shown in FIG. 12B. This state is maintained for a one-way propagation time T of pressure wave in the ink chamber 205 (pulse width of the first drive pulse 110a). This effects supply of the ink from the ink channel 213 during the maintenance of the above-described state.
The one-way propagation time T is required for the pressure wave in the ink chamber 205 to propagate in the direction of length of the ink chamber 205. The one-way propagation time T depends upon the length L (see FIG. 9B) of the ink chamber 205 and sound speed a in the ink in the ink chamber 205, that is, T=L/a. According to the theory of pressure wave propagation, the pressure in the ink chamber 205 is changed to positive pressure upon elapse of the time T from the time of application of the drive pulse 110a. The drive voltage applied to the electrode 204 of the ink chamber 205 is returned to zero in synchronism with the change to the positive pressure (at time T2).
Then, each shear mode wall actuator 203 is returned to the former state (see FIG. 9A), whereupon pressure is applied to the ink. Since pressure resulting from the return of each shear mode wall actuator 203 to the former state is added to the above-mentioned positive pressure, a relatively high pressure is induced in the vicinity of the nozzle 206 of the ink chamber 205. Consequently, the meniscus 230 is ejected as ink droplets 232 from the nozzle 206 at a predetermined speed, as shown in FIG. 12A. After the ejection, another meniscus comes out of the opening of the nozzle as shown in FIG. 12A.
Subsequently, a second drive pulse 110b is applied to the electrode 204 upon elapse of the one-way propagation time T from the fall of the first drive pulse 110a, that is, at time T3 so that ink droplets are ejected. The second drive pulse 110b has the same peak value (amplitude) as the first drive pulse 110a and a pulse width equal to the one-way propagation time T. The drive pulse signal 110 is thus applied to the recording head 21 in synchronism with input of the record data so that the ink droplets are ejected onto a recording medium such as recording paper, whereby recording is executed.
The viscosity of the ink used in the ink jet recording apparatus changes according to an ambient temperature. FIG. 8 shows the relationship between the ambient temperature of the ink and the ink viscosity. For example, the viscosity of the ink is about 3 mPa.multidot.s at the temperature of 25.degree. C. However, the viscosity changes to about 6 mPa.multidot.s at 10.degree. C. and about 2 mPa.multidot.s at 40.degree. C. as shown in FIG. 8.